The use of a stylus with a touch screen interface is well established. Touch screen designs have incorporated many different technologies including resistive, capacitive, inductive, and radio frequency sensing arrays. Resistive touch screens, for example, are passive devices well suited for use with a passive stylus. Although resistive touch screens can sense the input from nearly any object, multi-touch is generally not supported. An example of a multi-touch application may be applying two or more fingers to the touch screen. Another example may be inputting a signature, which may include simultaneous palm and stylus input signals. Due to these and other numerous disadvantages, capacitive touch screens are increasingly replacing resistive touch screens in the consumer marketplace.
Various tethered active stylus approaches have been implemented for use with touch screens and are found in many consumer applications such as point-of-sale terminals (e.g., the signature pad used for credit card transactions in retail stores) and other public uses. However, the need for a tethered cable is a significant drawback for private applications such as personal computers (“PCs”), smart phones, and tablet PCs.
Untethered active stylus approaches are also seen in various consumer applications. In an untethered active stylus approach, the stylus may be wirelessly synchronized to a capacitive sense array. A transmitter in or coupled to the capacitive sense array provides a synchronization signal that is received by a receiver in the stylus. The transmitter may wirelessly couple the synchronization signal in a variety of ways including, inductance, radio frequency, optical, ultrasound or other mediums. The stylus receives the synchronization signal from the transmitter and generates a transmit signal, based on the synchronization signal, which is detected by the capacitive sense array. Sensing circuitry in or coupled to the capacitive sense array can detect the presence of the stylus and determine a location of the stylus based on the detected transmit signal from the stylus. Keeping synchronicity between the capacitive sense array and the stylus imposes a high burden on the system, as it has impacts on battery life, cost and design-in support.